In recent years, as optical transfer systems have become larger in scale, efforts are exerted to miniaturize optical modules structuring those systems. An optical module includes an optical modulator chip and an electrical interface. As the optical modulator chip becomes more compact, the size of the electrical interface becomes a factor that determines the size of the optical module. Elements of the electrical interface can be divided into Radio Frequency (RF) pins connected to RF electrodes for high-speed signals and Direct Current (DC) pins connected to DC electrodes for controlling the optical modulator chip. In other words, RF signals are input from the RF pins provided on a lateral face or a bottom face of a package and are input to the RF electrodes of the optical modulator chip via a relay substrate. Similarly, DC signals are input from the DC pins provided on a lateral race or a bottom face of the package and are input to the DC electrodes of the optical modulator chip via a relay substrate.
As for the electrical interface, four streams of signals at 32 Gbps are input to the RF pins. Thus, the package is provided with four RF pins. However, when the distances between the four RF pins are shortened to miniaturize the optical module, crosstalk between the RF pins (channels) increases. Accordingly, to miniaturize the electrical interface, it is important how much the distances between the DC pins can be shortened, while keeping the RF pins at such distances from one another that satisfy characteristic impedance.
For example, twelve DC pins are arranged in a row in the width direction on a Flexible Printed Circuit (FPC) provided for the package. In this arrangement where the DC pins are arranged in a row in the width direction on the FPC, however, when the DC pins are soldered to land parts formed around through holes into which the DC pins are inserted, there is a possibility that the land parts positioned adjacent to each other may have short-circuits caused by the solder.
To cope with this situation, a structure has been proposed in which an FPC is provided with cut-out parts formed in such areas thereof that are positioned between adjacently-positioned land parts, so that the FPC can be shaped in such a manner that the adjacently-positioned land parts are at mutually-different heights. By using this structure, it is possible to configure an optical module so that the DC pins are soldered to the corresponding land parts, while the adjacently-positioned land parts are apart from each other in the height direction. Consequently, it is possible to inhibit the occurrence of crosstalk between the DC pins.